A Schottky barrier diode is a diode having a rectifying function utilizing a potential barrier formed on the junction surface of a Schottky metal having a sufficiently high carrier concentration and a semiconductor. For example, when the work function of the metal is taken as φm and the work function of the n-type semiconductor is taken as φs (where the work function of the semiconductor is defined as the difference between the vacuum level and the Fermi level), if a metal and a semiconductor that satisfy relationship |φm|>|φs| are brought into contact with each other, electrons present near the contact interface of the semiconductor move towards the metal so that the Fermi level of the metal coincides with the Fermi level of the semiconductor, and a depletion region is formed at the contact interface of the semiconductor, and at the same time, a potential barrier is formed at the metal-semiconductor interface. In this case, it is a diode with a metal side as a positive electrode and a semiconductor side as a negative electrode. At the time of application of forward bias, the potential barrier lowers, and electrons flow across the barrier, whereby current flows. At the time of application of reverse bias, electrons are blocked by the potential barrier and current flow is blocked. As the semiconductor, Si is most commonly used.
A Si-based Schottky diode is used in a high-speed switching device, a transmission/receiving mixer in a several GHz frequency band, a frequency conversion device or the like. It is generally used for power semiconductors. Since it has a small band gap of 1.1 eV and a small dielectric breakdown field of 0.3 MV/cm, there is a disadvantage that it is required to increase the thickness of the device in order to allow it to have a large withstand voltage, leading to an increase in forward on-resistance. In addition, a Si-based Schottky barrier diode having a high-speed response has an insufficient withstand voltage.
A Schottky diode utilizing SiC is also known. SiC has a large band gap of 3 eV or more and has a large dielectric breakdown field of 3 MV/cm. Therefore, a SiC-based Schottky diode is suited to power semiconductors, and active studies have been made on application of a SiC-based Schottky diode to power semiconductors. However, since it is difficult to fabricate a good crystal substrate, and epitaxial growth requires high-temperature processes, use of SiC has problems in respect of mass productivity and cost.
β-Ga2O3 has a further wider band gap (4.8 eV to 4.9 eV), and hence, is expected to have a high withstand voltage. However, it has a problem in producing a good substrate as well as in mass productivity and cost.
An oxide semiconductor has a wide band gap as compared with Si and has a high dielectric breakdown field, and therefore, it is expected to be applied to power semiconductors. In particular, a Schottky barrier diode is expected to have a high-speed response and excellent reverse recovery characteristics.
Non-Patent Document 1 discloses a Schottky barrier diode in which amorphous IGZO is used as an oxide semiconductor and a Ti/Pd laminated body is used as a Schottky metal electrode. In this technology, by subjecting Pd to an oxygen plasma treatment, an excellent Schottky barrier is formed. However, this technology is a diode in which current is extracted in the lateral direction, and it was difficult to extract large current due to resistance of an extraction electrode. Further, the electrode positioned at the lower end of the oxide semiconductor layer plays a role of a Schottky electrode, and the conduction direction is opposite to that of a Schottky barrier diode using common Si or SiC. When incorporating this Schottky barrier diode in a conventional electronic circuit, there was a problem in compatibility with other electronic materials. Also in the case of extracting the current in the lateral direction, in this technology, leakage current in the reverse direction is large. Therefore, when this is incorporated into an electronic circuit in which a Schottky barrier diode is used, there is a concern that power loss at the time of output becomes large as compared with the input power or a circuit itself malfunctions.
Patent Document 1 discloses a Schottky barrier diode in which a Ga2O3-based compound semiconductor is used as an oxide semiconductor layer and the oxide semiconductor layer is disposed between an ohmic electrode layer and a Schottky electrode layer. However, if a Ga2O3-based semiconductor layer is formed on a silicon substrate, for example, a forward on-resistance is increased, and when this is incorporated into an electric circuit in which a Schottky barrier diode is used, power loss at the time of output becomes large as compared with input power.
Patent Document 2 discloses a technology in which a gate electrode and a source or drain electrode of FET using an oxide semiconductor are electrically connected, realizing a diode having a small reverse saturated current. However, by this method, the device structure becomes complicated, resulting in poor yield when fabricated into a device.